There is two 1/8's to make 1/4. This is proven by the following example:
1/8 + 1/8 = 2/8.
2/8 (simplified) = 1/4.
Given the information above, we can safely figure out this problem.
Since 2/8 make 1/4, and we're looking for 3/4, multiply 2/8 by 3.
2/8 x 3 = 6/8.
It will require 6 total 1/8's.
I hope this helps!